While some have billed the test as an early birthday present for Kim, we look at strange – but possibly true – facts about one of the world's most secretive heads of state and his country.

Kim Jong-un is the world's youngest head of state, believed to be turning 33 on 8 January – but nobody really knows his true age.

He was born to his late father Kim Jong-Il and his second mistress Ko Young Hee.

He is the second of three children. He has an elder brother called Kim Jong-chul and a younger sister called Kim Yo-jong.

Despite no formal military training, Kim Jong-un is head of one of the largest armies in the world. According to the US State Department, DPRK has an estimated active duty military force of up to 1.2 million personnel.

The only known information on his early life comes from defectors and those who have claimed to see him abroad, such as his education in Switzerland.

North Korean leader Kim Jong Un issues instructions while visiting the construction site of the Mirae Scientists Street in Pyongyang, North KoreaReuters

According to reports, he attended a private English-language International school near Bern, Switzerland.

North Korean leader Kim Jong Un watches a performance, "Song of Youth Following the Sun", given by the Art Squad of the Central Committee of the Kim Il Sung Socialist Youth League in Pyongyang, North KoreaReuters

The number of North Korean children being treated for severe malnutrition increased more than 30% in 2014, according to a United Nations agency.